Study On Boiling Heat Transfer Enhancement Of Self-assembled Carbon-based Nanomaterials Coating Surfaces

With the rapid development of electronic technology and the communication industry,the heat dissipation problems of 5G communications and high-power-density electronic devices are becoming more severe.There is an urgent need to develop high-efficiency and low-energy boiling heat transfer technology.At the same time,enhanced boiling heat transfer technology can effectively reduce cooling energy consumption,cooling electricity and carbon emissions,and is of great significance to achieving the goals of "carbon compliance" and "carbon neutrality",A large number of studies have shown that carbon nanomaterials coatings can effectively enhance the boiling heat transfer performance because of their excellent thermal conductivity,chemical properties and mechanical strength.It is an efficient method to enhance boiling phase change heat transfer and has strong potentials for industrial applications.The nucleate boiling self-assembly method is demonstrated to be an ideal method for preparing carbon nanomaterials coatings due to its simple steps,low cost and ease to prepare large-scale coatings.In this paper,for improving the nucleate boiling heat transfer performance,self-assembled graphene oxide(GO)nanocoatings and multi-walled carbon nanotubes(MWCNTs)coatings were prepared by the facile and cost-effective nucleate boiling self-assembly method.And by systematically exploring the saturated pool boiling heat transfer characteristics and bubble dynamics of GO nanocoatings and MWCNTs nanocoatings with different deposition parameters,the optimal deposition parameters,and the GO nanocoating and MWCNTs nanocoating with optimal nucleate boiling heat transfer performance were obtained.Besides,the boiling heat transfer enhancement mechanisms of GO nanocoatings and MWCNTs nanocoatings were analyzed,and theoretical prediction models of critical heat flux(CHF)for the two nanocoatings were established respectively.At first,a visualizing high-heat-flux experimental platform was designed and built.Its highest heating heat flux was found to be nearly 1000 W·cm-2 under safe conditions,which can meet the heating heat flux requirements of various micro/nano scale phase change heat transfer experiments.Furthermore,the reliability of the experimental platform and method were verified by the visualized boiling heat transfer experiment of the plain copper surface.Using the visualizing high-heat-flux experimental platform,the self-assembled GO nanocoating was successfully fabricated on a copper substrate by utilizing the nucleate boiling self-assembly method.Pool boiling experimental results showed that CHF and maximum heat transfer coefficient(HTC)were 78%and 41%respectively higher than that on the pristine plain copper surface.And CHF increased linearly with the increasing subcooling degree,but HTC decreased slightly.The bubble visualization showed that,as compared to the pristine plain copper surface,on the GO nanocoating,the bubble departure diameter decreased but the departure frequency and nucleation site density increased.Moreover,the bubble coalescence was slower.Under the subcooled boiling,the microbubble jets occurred on the GO nanocoating at medium and high heat flux regions.The amount and jet frequency of microbubbles increased with the increasing subcooling degree.The effect of deposition parameters on boiling heat transfer performance of GO nanocoatings was also systematically explored.It was found that the optimal deposition heat flux,deposition time and deposition concentration were 100 W·cm-2,2.5 h and 1.6×10-4 wt.%,respectively.The GO nanocoating with deposition time of 2.5 h got the best boiling performance among all GO nanocoatings with a CHF of 261 W·cm-2 and a maximum HTC 9.1 W·cm-2·K-1.Repeated boiling experiments demonstrated little differences of HTC on the optimal GO nanocoating with the number of boiling and no shedding was observed,indicating good stability and durability.Differing from GO nanosheets,MWCNTs have a unique fiber-like structure,which makes them easy to form porous network coating during self-assembly.This structure is conducive to the improvement of HTC and has a unique advantage in heat transfer enhancement.Therefore,MWCNTs nanocoatings with different deposition parameters were also fabricated by utilizing the nucleate boiling self-assembly method.It found that the optimal deposition concentration was 4.0×10-5 wt.%and the optimal deposition time was about 50 minutes.Then,MWCNTs nanocoating with the optimal deposition time and deposition concentration was fabricated.Its CHF and maximum HTC were 218 W·cm-2 and 10.7 W·cm-2·K-1 respectively,which were 87%and 73%higher,and the wall superheat of the boiling incipience(ONB)was 38%lower than those of the pristine plain copper surface.The bubble visualization results showed that as compared to the pristine plain copper surface,its bubble departure diameter decreased,but the departure frequency increased,and the nucleation site density increased by 1.5 times.Furthermore,the bubble merging is slower and isolated bubbles were observed on the optimal MWCNTs nanocoating near CHF.Also,20 repeated boiling experiments showed that the MWCNTs nanocoating has good stability and durability.When compared with the optimal GO nanocoating,the CHF of the optimal MWCNTs nanocoating was reduced by 17%,but HTC increased by?30%at a similar given heat flux.It is indicated that the closely packed two-dimensional GO nanocoating with high thermal conductivity is more conducive to greatly increase CHF,while MWCNTs nanoporous coating can highly enhance HTC.Then,the self-assembled hybrid GO/MWCNTs nanocoating was prepared,whose CHF and maximum HTC reached 230 W·cm-2 and 12.4 W·cm-2·K-1,respectively.This exhibited combined advantages of GO nanocoating and MWCNTs nanocoating and greatly increased CHF and HTC simultaneously.At the same given heat flux,the maximum incremental ratios of HTC of hybrid GO/MWCNTs nanocoating were 46%and 40%respectively when compared with the optimal GO nanocoating and the optimal MWCNTs nanocoating.Additionally,good stability and durability of the hybrid GO/MWCNTs nanocoating were also verified by repeated boiling experiments.To better understand the boiling heat transfer enhancement mechanisms of GO nanocoatings and MWCNTs nanoporous coatings,surface charateristics of two kinds of nanocoatings were characterized.Combined with the results of bubble visualization,it was found that the high thermal conductivity of the two-dimensional GO film coating,the increased wettability and surface roughness,and the micro/nano scale bump structures resulting from the partially folded edges of GO nanosheets,worked together to make the nucleate boiling heat transfer performance improved.However,the MWCNTs nanoporous coating effectively increased the nucleation site density and induced capillary wicking effect,as well as increased the wettability and surface roughness.These parameters together augmented the nucleate boiling heat transfer performance.Finally,by taking all the influence factors of the boiling heat transfer performance into consideration,the existing CHF theoretical model was modified and new CHF prediction models of self-assembled GO nanocoating and MWCNTs nanocoating were established.It was found that the CHF predicted values and experimental values of all GO nanocoatings and MWCNTs nanocoatings were within±13%and ±8%,respectively.It can be seen that both theoretical models can predict experimental CHF well.In this paper,a gradual and in-depth study on the nucleate boiling heat transfer enhancement of GO nanocoatings and MWCNTs nanocoatings was carried out.The optimal deposition parameters,as well as the GO nanocoating and MWCNTs nanocoating with optimal nucleate boiling heat transfer performance were found.Moreover,the boiling heat transfer enhancement mechanisms of GO nanocoatings and MWCNTs nanocoatings were revealed.By comprehensive consideration of various influencing factors,two prediction models of CHF on the two coating surfaces were also established respectively,and they were verified to predict CHF well.This paper provides an effective solution to solve the heat dissipation problem of higher heat flux density electronic devices with lower energy consumption.It also provides a theoretical basis for the design of advanced thermal management systems for high heat flux density electronic devices based on nanocoating technologies.